Jonathan D. Webb

A Heterogeneous Nickel Catalyst for the Hydrogenolysis of Aryl Ethers without Arene Hydrogenation

A heterogeneous nickel catalyst for the selective hydrogenolysis of aryl ethers to arenes and alcohols generated without an added dative ligand is described. The catalyst is formed in situ from the well-defined soluble nickel precursor Ni(COD)(2) or Ni(CH(2)TMS)(2)(TMEDA) in the presence of a base additive, such as (t)BuONa. The catalyst selectively cleaves C(Ar)-O bonds in aryl ether models of lignin without hydrogenation of aromatic rings, and it operates at loadings down to 0.25 mol % at 1 bar of H(2) pressure. The selectivity of this catalyst for electronically varied aryl ethers differs from that of the homogeneous catalyst reported previously, implying that the two catalysts are distinct from each other.

Advancing the Mechanistic Understanding of an Enantioselective Palladium-Catalyzed Alkene Difunctionalization Reaction

The mechanism of an enantioselective palladium-catalyzed alkene difunctionalization reaction has been investigated. Kinetic analysis provides evidence of turnover-limiting attack of a proposed quinone methide intermediate with MeOH and suggests that copper is involved in productive product formation, not just catalyst turnover. Through examination of substrate electronic effects, a Jaffé relationship was observed correlating rate to electronic perturbation at two positions of the substrate. Ligand effects were evaluated to provide evidence of rapid ligand exchange between palladium and copper as well as a correlation between ligand electronic nature and enantioselectivity.

N-Heterocyclic Carbene Complexes of Rh: Reaction With Dioxygen Without Oxidation

The reaction of oxygen with rhodium complexes containing N-heterocyclic carbenes was found to give dioxygen complexes with rare square planar geometries and unusually short O-O bond lengths. Analysis of the bonding in these complexes by Rh L-edge X-ray absorption spectroscopy (XAS), Raman spectroscopy, and DFT calculations provides evidence for a bonding model in which singlet oxygen is bound to a Rh(I) d8 metal complex, rather than the more common Rh(III) d6 peroxo species with octahedral geometry and O-O bond lengths in the 1.4-1.5 A range.

Chemo- and Regioselective Hydrogenolysis of Diaryl Ether C−O Bonds by a Robust Heterogeneous Ni/C Catalyst: Applications to the Cleavage of Complex Lignin-Related Fragments

We report the chemo- and regioselective hydrogenolysis of the C-O bonds in di-ortho-substituted diaryl ethers under the catalysis of a supported nickel catalyst. The catalyst comprises heterogeneous nickel particles supported on activated carbon and furnishes arenes and phenols in high yields without hydrogenation. The high thermal stability of the embedded metal particles allows C-O bond cleavage to occur in highly substituted diaryl ether units akin to those in lignin. Preliminary mechanistic experiments show that this catalyst undergoes sintering less readily than previously reported catalyst particles that form from a solution of [Ni(cod)2].

Modified mesoporous materials as Pd scavengers and catalyst supports

The use of mesoporous molecular sieves (MMSs) modified by mercaptopropyl trimethoxysilane (MPTMS) to scavenge Pd is described. The ordered mesoporous material displays excellent ability to remove Pd from organic and aqueous solutions. After only one treatment, a 500-ppm solution of PdCl2 in water can be reduced to 1 ppb. In addition, the resulting material is an effective, leach-proof catalyst for the Suzuki-Miyaura and Mizoroki-Heck reactions. Thus, the Suzuki-Miyaura reaction can be conducted in water at 80 °C with as little as 3 ppb Pd leaching. Hot filtrations and three-phase tests confirm that the catalyst acts without leaching Pd from the surface.

Borohydrides from Organic Hydrides: Reactions of Hantzsch's Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzschs ester analogues 1 a-d with the Lewis acidic species B(C(6)F(5))(3) results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a-d, which are obtained in high to moderate yields. The N-substituted substrates (N-Me, N-Ph) reacted in high yield 90-98 % and the connectivity of the products were confirmed by an X-ray crystallographic analysis of the N-Me borohydride salt 2 a. Unsubstituted Hanztschs ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester-bound Lewis acid-base adduct 3 a. Formation of the Lewis acid-base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d. The connectivity of the carbonyl-bound adduct was confirmed by an X-ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C(6)F(5))(3). We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C(6)F(5))(3) with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid-base adduct of the starting material and B(C(6)F(5))(3). The 1,2-dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid-base adduct.

The Synthesis of Chiral Periodic Organosilica Materials with Ultrasmall Mesopores

Helical organosilica materials were synthesized for the first time using a novel binaphthyl-based chiral co-monomer in less than 1 hour. The incorporation of a chiral co-monomer in the wall was shown to influence the curvature of the helical materials. As the amount of the chiral co-monomer was increased, the degree of curvature increased, illustrating the importance of this monomer to the overall morphology.

Thioboration of α,β-unsaturated ketones and aldehydes toward the synthesis of β-sulfido carbonyl compounds.

Herein a direct β-sulfido carbonyl compound synthesis by the easy activation of RS-Bpin reagents with α,β-unsaturated ketones and aldehydes is reported. This convenient methodology can be performed at room temperature with no other additives. The key point of this reactivity is based on the Lewis acidic properties of the boryl unit of the RS-Bpin reagent interacting with the C═O oxygen. Consequently, the SR unit becomes more nucleophilic and promotes the 1,4- versus the 1,2-addition, as a function of the involved substrate. The thioborated products can be further transformed into β-sulfido carbonyl compounds by addition of MeOH.